The Third Generation Partnership Project (3GPP) defines a long-term evolution (LTE) architecture, which provides high data rate, low latency, packet optimization and improved system capacity and coverage. In an LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred to as user equipments (UEs).
An LTE network supports only packet-switched (PS) services. However fallback is specified for circuit switched (CS) services as well. CSFB defines a mechanism for using a legacy CS network (e.g. Universal Mobile Telecommunications System Terrestrial Radio Access Network (UTRAN) or a GSM EDGE Radio Access Network (GERAN)) to provide voice and traditional CS-domain services (e.g. voice call, a CS short message service (SMS), Unstructured Supplementary Service Data (USSD), among others).
With CSFB, when there in an incoming CS call for a UE operating in LTE mode, the LTE network pages the device. The device responds with a special service request message to the network, and the network signals the device to move (fall back) to GERAN/UTRAN cell by providing redirection information for the UE to acquire a UTRAN/GERAN cell. Similarly for outgoing CS calls, the same special service request is used to move the device to GERAN/UTRAN to place the outgoing call. The UE then attempts to acquire a GERAN or UTRAN cell based on the received redirection information and then attempts to establish a connection.
The UE then attempts to establish CS service and may attempt to move packet switched services from E-UTRAN to GERAN or UTRAN if applicable. In particular, to maintain data connectivity, an always-on UE is required to perform a Routing Area Update (RAU) procedure to transfer the PS data context from E-UTRAN to UTRAN/GERAN. In addition, there may have been a data session in progress when the CSFB call was initiated, which would trigger the UE to perform an RAU procedure and then initiate a packet data call in parallel with the establishment of the CS call.
The overall call setup time for a CSFB call is significantly longer compared to a CS voice call that is initiated while the UE is already in GERAN/UTRAN. Further, additional delays are caused if PS domain signaling activities take place while the CS domain is being setup.